There has been known an exhaust gas purification apparatus having a NOx selective catalytic reduction catalyst (hereinafter, referred to simply as a “NOx catalyst”) which removes NOx contained in exhaust gas discharged from an internal combustion engine by using ammonia as a reducing agent. In the exhaust gas purification apparatus, there is provided a supply valve or the like which serves to supply ammonia or a precursor of ammonia into the exhaust gas at a location upstream from the NOx catalyst, so that an ammonia adsorption amount in the NOx catalyst can be in a state adequate for removal of NOx.
Here, in a technology disclosed in a first patent literature, in cases where an actual NOx removal rate calculated based on detected values from NOx sensors, which are disposed upstream and at downstream side of a NOx catalyst, respectively, does not reach a target NOx removal rate correlated with the temperature of the NOx catalyst, adjustment is made to increase the addition amount of urea water as a precursor of ammonia. At this occasion, if the NOx removal rate after the increasing adjustment is not increased from the NOx removal rate before the increasing adjustment, it is judged that an ammonia slip is occurring in the NOx catalyst, and then, under other predefined conditions, it will be judged that an exhaust gas purification apparatus is in failure.